Semiconductor device with multiple hbts having different emitter ballast resistances

ABSTRACT

The present disclosure relates to a semiconductor device with multiple heterojunction bipolar transistors (HBTs) that have different emitter ballast resistances. The disclosed semiconductor device includes a substrate, a first HBT and a second HBT formed over the substrate. The first HBT includes a first collector, a first base over the first collector, a first emitter over the first base, and a first cap structure over the first emitter. The second HBT includes a second collector, a second base over the second collector, a second emitter over the second base, and a second cap structure over the second emitter. Herein, the first cap structure is different from the second cap structure, such that a first emitter ballast resistance from the first cap structure is at least 1.5 times greater than a second emitter ballast resistance from the second cap structure.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 62/296,128, filed Feb. 17, 2016, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a semiconductor device with multipleheterojunction bipolar transistors (HBTs) and more particularly to asemiconductor device with multiple HBTs that have different emitterballast resistances for improved thermal stability performance.

BACKGROUND

Transistors are an essential component in modern mobile communicationsdevices. Specifically, transistors play a vital role in the transmissionand reception of radio frequency (RF) signals in the front end of amobile communications device. Due to the decreasing form factor ofmobile communications devices, the desire for a longer battery life, andsupport for an increasing number of stringent wireless communicationsstandards, there is an ongoing need for smaller, more efficienttransistor devices with improved performance characteristics.

As will be appreciated by those of ordinary skill in the art, one way toimprove the performance at high frequencies (e.g., radio frequencies) isby using heterojunction bipolar transistors (HBTs). At high frequencies,HBTs offer many performance advantages over homojunction bipolartransistors. The performance advantages offered by conventional HBTsprimarily arise due to a wider energy bandgap in the material of theemitter of the device as compared to the energy bandgap in the materialof the base of the device. The wider energy bandgap of the emittermaterial allows for many parameters dictating the performance of thedevice to be optimized for high frequencies without degrading thecurrent gain of the device.

For high power applications, multiple HBTs may be integrated into onesemiconductor circuit to accommodate high currents as shown in FIG. 1.In some conditions, one of the HBTs, like HBT1, may run hotter than theother HBTs, and draw more current resulting in thermal runaway. Onesolution to provide good thermal stability for HBT1-HBTn is to applyemitter ballast resistors R1-Rn in series with HBT1-HBTn, respectively.These emitter ballast resistors R1-Rn may be provided by conventionalthin film resistors, which may, however, significantly increase the sizeof the semiconductor circuit. Further, for some packaging technology,like flip-chip technology, emitters e1-en of the HBT1-HBTn will beconnected together (not shown), which basically shorts out the emitterballast resistors R1-Rn. As such, the HBT1-HBTn will not have differentemitter ballast resistors to improve the thermal stability of thedevice.

Accordingly, there remains a need for improved semiconductor devicedesigns to utilize the advantages of HBTs while achieving structureflexibilities such that HBTs may have different emitter ballastresistances for improved thermal stability performance. In addition,there is also a need to keep the size and cost of the final productseffective.

SUMMARY

The present disclosure relates to a semiconductor device with multipleheterojunction bipolar transistors (HBTs) that have different emitterballast resistances for improved thermal stability performance. Thedisclosed semiconductor device includes a substrate, a first HBT, and asecond HBT formed over the substrate. The first HBT includes a firstcollector, a first base formed over the first collector, a first emitterformed over the first base, a first cap structure formed over the firstemitter, and a first emitter contact connected to the first capstructure. The second HBT includes a second collector, a second baseformed over the second collector, a second emitter formed over thesecond base, a second cap structure formed over the second emitter, anda second emitter contact connected to the second cap structure. Herein,the first cap structure is different from the second cap structure, suchthat a first emitter ballast resistance from the first cap structurebetween the first emitter contact and the first emitter is at least 1.5times greater than a second emitter ballast resistance from the secondcap structure between the second emitter contact and the second emitter.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 shows multiple paralleling heterojunction bipolar transistors(HBTs) with emitter ballast resistors.

FIG. 2 shows an exemplary semiconductor device with multiple HBTs thathave different emitter ballast resistances according to one embodimentof the present disclosure.

FIG. 3 shows an exemplary semiconductor device with multiple HBTs thathave different emitter ballast resistances according to one embodimentof the present disclosure.

FIG. 4 shows an exemplary semiconductor device with multiple HBTs thathave different emitter ballast resistances according to one embodimentof the present disclosure.

FIGS. 5A-5C provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device shown in FIG. 2.

FIGS. 6A-6D provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device shown in FIG. 3.

FIGS. 7A-7C provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device shown in FIG. 4.

It will be understood that for clear illustrations, FIGS. 2-7 may not bedrawn to scale.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to a semiconductor device with multipleheterojunction bipolar transistors (HBTs) that have different emitterballast resistances and a process for making the same. FIG. 2 shows anexemplary semiconductor device 10 with multiple HBTs that have differentemitter ballast resistances according to one embodiment of the presentdisclosure. For simplification, there are only two HBTs illustrated inthe semiconductor package 10. In practical applications, thesemiconductor package 10 may include 6 to 50 HBTs. Herein, thesemiconductor package 10 includes a substrate 12, a first HBT 14 and asecond HBT 16 formed over the substrate 12.

In detail, the substrate 12 may be semi-insulating and formed ofGallium-Arsenide (GaAs). The first HBT 14 includes a first subcollector18, a first collector 20 formed over the first subcollector 18, a firstbase 22 formed over the first collector 20, a first emitter 24 formedover the first base 22, and a first cap structure 26 formed over thefirst emitter 24. In addition, a pair of first collector contacts 28 areformed over the first subcollector 18, a pair of first base contacts 30are formed over the first base 22, and a first emitter contact 32 isformed over the first cap structure 26.

The first subcollector 18 may be formed from Gallium-Arsenide (GaAs)with a doping concentration between 1 e18/cm³ and 2e19/cm³. The firstcollector 20 may be formed from GaAs with a doping concentration between1 e15/cm³ and 2e17/cm³. In different applications, both GaAs andAluminum-Gallium-Arsenide (AlGaAs) may be used to form the firstcollector 20, and a gradient doping concentration may be used in thefirst collector 20 as well. The first base 22 may be formed fromGallium-Arsenide (GaAs) with a doping concentration between 7e18/cm³ and1 e20/cm³. The first emitter 24 may be formed fromIndium-Gallium-Phosphide (In_(x)Ga_(1-x)P) with a doping concentrationbetween 5e16/cm³ and 5e17/cm³. Herein ‘x’ has a value between 0.4 and0.6 representing relative quantities of respective elements. The firstcollector contacts 28, the first base collector contacts 30, and thefirst emitter contact 32 are formed from metals. The first subcollector18 has a thickness between 5000 Å and 10000 Å, the first collector 20has a thickness between 3000 Å and 12000 Å, the first base 22 has athickness between 500 Å and 1600 Å, and the first emitter 24 has athickness between 300 Å and 700 Å.

In this embodiment, the first cap structure 26 includes a first lowercap 34 formed over the first emitter 24, a first middle cap 36 formedover the first lower cap 34, a first ballast resistor layer 38 formedover the first middle cap 36, and a first upper cap 40 formed over thefirst ballast resistor layer 38. The first lower cap 34 may be formedfrom GaAs with a doping concentration between 1 e17/cm³ and 1 e19/cm³;the first middle cap 36 may be formed from GaAs compositionally gradedinto Indium-Gallium-Arsenide (InGaAs) from a bottom to a top of thefirst middle cap 36 with a doping concentration greater than 1 e19/cm³;and the first upper cap 40 may be formed from InGaAs with a dopingconcentration greater than 1 e19/cm³.

The first ballast resistor layer 38 may be formed from any lattice-matchmaterial to InGaAs with a wide bandgap. One exemplary material used inthe first ballast resistor layer 38 is Indium-Aluminum-Arsenide(In_(0.5)Al_(0.5)As) (i.e., a mixture of approximately 50%Indium-Arsenide and 50% Aluminum-Arsenide). Another exemplary materialused in the first ballast resistor layer 38 is Indium-Phosphide (InP).In addition, In_(y)Ga_((1-y))As with a different composition of In_(y)Asand Ga_((1-y))As than the first upper cap 40 may also be used in thefirst ballast resistor layer 38. Herein ‘y’ represents relativequantities of the Indium element. In this embodiment, ‘y’ having a valueless than 0.5 may provide the first ballast resistor layer 38 a widerbandgap with additional constraints of engineering the composition andthickness to make the first ballast resistor layer 38 pseudomorphic. Thefirst ballast resistor layer 38 is provided as an emitter ballastresistor integrated within the first HBT 14 between the first emitter 24and the first emitter contact 32. A doping concentration of the firstballast resistor layer 38 is relatively low between 1 e17/cm³ and 1e19/cm³, because the first ballast resistor layer 38 is used as acarrier (electrons/holes) barrier. The resistance of the first ballastresistor layer 38 may also be controlled by its thickness. In thisembodiment, the first ballast resistor layer 38 has a thickness between10 Å and 1500 Å.

The second HBT 16 has a similar configuration to the first HBT 14. Thesecond HBT 16 includes a second subcollector 42, a second collector 44formed over the second subcollector 42, a second base 46 formed over thesecond collector 44, a second emitter 48 formed over the second base 46,and a second cap structure 50 formed over the second emitter 48. Inaddition, a pair of second collector contacts 52 are formed over thesecond subcollector 42, a pair of second base contacts 54 are formedover the second base 46, and a second emitter contact 56 is formed overthe second cap structure 50.

The first subcollector 18 and the second subcollector 42 may be formedfrom a common subcollector layer, the first collector 20 and the secondcollector 44 may be formed from a common collector layer, the first base22 and the second base 46 may be formed from a common base layer, andthe first emitter 24 and the second emitter 48 may be formed from acommon emitter layer. Herein, forming from a common(subcollector/collector/base/emitter) layer refers to forming from acommon epitaxial layer with a same material composition, a same dopingconcentration, and a same thickness, which may be broken intodiscontinuous segments during fabrication. In some applications, thecommon subcollector layer is a contiguous subcollector layer (notshown).

The second cap structure 50 is different from the first cap structure 26and includes a second lower cap 58 and a second middle cap 60 without aballast resistor layer. The second lower cap 58 may be formed from GaAswith a doping concentration between 1 e17/cm³ and 1 e19/cm³; and thesecond middle cap 60 may be formed from GaAs compositionally graded intoInGaAs from a bottom to a top of the second middle cap 60 with a dopingconcentration greater than 1 e19/cm³. The first lower cap 34 and thesecond lower cap 58 may be formed from a common lower cap layer, and thefirst middle cap 36 and the second middle cap 60 may be formed from acommon middle cap layer. Herein, forming from a common (lower/middle)cap layer refers to forming from a common epitaxial layer with a samematerial composition, a same doping concentration, and a same thickness,which may be broken into discontinuous segments during fabrication.

Because the first cap structure 26 includes the first ballast resistorlayer 38, which is used as a carrier (electrons/holes) barrier, and thesecond cap structure 50 does not include a ballast resistor layer, afirst emitter ballast resistance from the first cap structure 26 is atleast 1.5 times greater than a second emitter ballast resistance fromthe second cap structure 50. The chosen first emitter ballast resistanceis dependent on the specific application in which the first HBT 14 isused. The first emitter ballast resistance is formed between the firstemitter 24 and the first emitter contact 32, and the second emitterballast resistance is formed between the second emitter 48 and thesecond emitter contact 56.

In order to get a relatively low emitter ballast resistance compared tothe first emitter ballast resistance of the first HBT 14, anotherconfiguration of a second HBT 16L is provided as illustrated in FIG. 3.The second HBT 16L has a similar configuration to the first HBT 14. Thesecond HBT 16L includes a second cap structure 50L, which includes thesecond lower cap 58, the second middle cap 60, a second ballast resistorlayer 62, and a second upper cap 64. The second ballast resistor layer62 may be formed from the same material composition with the same dopingconcentration and thickness as the first ballast resistor layer 38. Thesecond upper cap 64 and the first upper cap 40 may be formed from acommon upper cap layer. Herein, forming from a common upper cap layerrefers to forming from a common epitaxial layer with a same materialcomposition, a same doping concentration, and a same thickness, whichmay be broken into discontinuous segments during fabrication.

The difference from the first HBT 14 is that the second HBT 16L includesa lengthened second emitter contact 56L, which is not formed over thesecond cap structure 50L, but extends through the second upper cap 64and the second ballast resistor layer 62, and extends into the secondmiddle cap 60.

Because the lengthened second emitter contact 56L is formed from analloyed metal, the second upper cap 64 and the second ballast resistorlayer 62 are shorted. As such, the first emitter ballast resistance ofthe first HBT 14 between the first emitter 24 and the first emittercontacts 32 is at least 1.5 times greater than a second emitter ballastresistance of the second HBT 16L between the second emitter 48 and thelengthened second emitter contact 56L.

FIG. 4 shows another exemplary semiconductor device 10NB with multipleHBTs that have different emitter ballast resistances according to oneembodiment of the present disclosure. The semiconductor package 10NBincludes the substrate 12, a first HBT 14NB and a second HBT 16NB formedover the substrate 12, where neither the first HBT 14NB nor the secondHBT 16NB includes a ballast resistor layer.

The first HBT 14NB includes the first subcollector 18, the firstcollector 20 formed over the first subcollector 18, the first base 22formed over the first collector 20, the first emitter 24 formed over thefirst base 22, and a first cap structure 26NB formed over the firstemitter 24. Herein, the first cap structure 26NB only includes the firstlower cap 34. In addition, the first collector contacts 28 are formedover the first subcollector 18, the first base contacts 30 are formedover the first base 22, and the first emitter contact 32 is formed overthe first lower cap 34.

The second HBT 16NB has a similar configuration to the first HBT 14NB.The difference from the first HBT 14NB is that the second HBT 16NB has asecond cap structure 50NB, which includes the second lower cap 58 formedover the second emitter 48, the second middle cap 60 formed over thesecond lower cap 58, and the second upper cap 64 formed over the secondmiddle cap 60 without a ballasted resistor layer. The second emittercontact 56 is formed over the second cap structure 50NB and in contactwith the second upper cap 64. Herein, the second lower cap 58 may beformed from a common lower cap layer as the first lower cap 34. Formingfrom a common lower cap layer refers to forming from a common epitaxiallayer with a same material composition, a same doping concentration, anda same thickness, which may be broken into discontinuous segments duringfabrication. The second middle cap 60 and second upper cap 64 may becomprised of a superlattice structure.

The first cap structure 26NB has a higher contact resistance than thesecond cap 50NB because the second lower cap 58 (the first lower cap 34)is designed to have a higher contact resistance than the second uppercap 64 due to lower doping concentration and a different materialcomposition from the second upper cap 64. In addition, the second middlecap 60 and second upper cap 64 may be comprised of the superlatticestructure, which is designed to give a low contact resistance to thesecond emitter cap 50NB. As such, a first emitter ballast resistance ofthe first HBT 14NB between the first emitter 24 and the first emittercontact 32 is at least 1.5 times greater than a second emitter ballastresistance of the second HBT 16NB between the second emitter 48 and thesecond emitter contact 56.

FIGS. 5A-5C provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device 10 shown in FIG. 2.Although the exemplary steps are illustrated in a series, the exemplarysteps are not necessarily order dependent. Some steps may be done in adifferent order than that presented. Further, processes within the scopeof this disclosure may include fewer or more steps than thoseillustrated in FIGS. 5A-5C.

Initially, a first no-contact HBT 14NC and a second no-contact HBT 16NCformed over the substrate 12 are provided as depicted in FIG. 5A. Thefirst no-contact HBT 14NC includes the first subcollector 18, the firstcollector 20 formed over the first subcollector 18, the first base 22formed over the first collector 20, the first emitter 24 formed over thefirst base 22, and the first cap structure 26 formed over the firstemitter 24. The first cap structure 26 includes the first lower cap 34formed over the first emitter 24, the first middle cap 36 formed overthe first lower cap 34, the first ballast resistor layer 38 formed overthe first middle cap 36, and the first upper cap 40 formed over thefirst ballast resistor layer 38.

The second no-contact HBT 16NC has a similar configuration to the firstno-contact HBT 14NC. The second no-contact HBT 16NC includes the secondsubcollector 42, the second collector 44 formed over the secondsubcollector 42, the second base 46 formed over the second collector 44,the second emitter 48 formed over the second base 46, and a secondbefore-etching cap structure 50′ formed over the second emitter 48. Thesecond before-etching cap structure 50′ includes the second lower cap 58formed over the second emitter 48, the second middle cap 60 formed overthe second lower cap 58, the second ballast resistor layer 62 formedover the second middle cap 60, and the second upper cap 64 formed overthe second ballast resistor layer 62. Herein, the second ballastresistor layer 62 may be formed from any lattice-match material toInGaAs with a wide bandgap. The second ballast resistor layer 62 may beformed of In_(0.5)Al_(0.5)As, InP or In_(y)Ga_((1-y))As and used as anetch stop layer in a later step.

Next, the second upper cap 64 and the second ballast resistor layer 62are removed from the second no-contact HBT 16NC to form a second etchedHBT 16E as depicted in FIG. 5B. The second lower cap 58 and the secondmiddle cap 60 are included in the second cap structure 50 of the secondetched HBT 16E. A wet/dry etchant chemistry, such as potassium hydroxide(KOH), sodium hydroxide (NaOH), and acetylcholine (ACH), may be used toremove the second upper cap 64 and the second ballast resistor layer 62.

Finally, the first collector contacts 28, the first base contacts 30,and the first emitter contact 32 are provided to the first no-contactHBT 14NC to form the first HBT 14, and the second collector contacts 52,the second base contacts 54, and the second emitter contact 56 areprovided to the second etched HBT 16E to form the second HBT 16 asdepicted in FIG. 5C. For the first HBT 14, the first collector contacts28 are formed over the first subcollector 18, the first base contacts 30are formed over the first base 22, and the first emitter contact 32 isformed over the first upper cap 40 of the first cap structure 26. Forthe second HBT 16, the second collector contacts 52 are formed over thesecond subcollector 42, the second base contacts 54 are formed over thesecond base 46, and the second emitter contact 56 is formed over thesecond middle cap 60 of the second cap structure 50.

FIGS. 6A-6D provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device 10L shown in FIG. 3.Although the exemplary steps are illustrated in a series, the exemplarysteps are not necessarily order dependent. Some steps may be done in adifferent order than that presented. Further, processes within the scopeof this disclosure may include fewer or more steps than thoseillustrated in FIGS. 6A-6D.

Initially, a first no-contact HBT 14NC and a second no-contact HBT 16NCformed over the substrate 12 are provided as depicted in FIG. 6A. Thefirst no-contact HBT 14NC includes the first subcollector 18, the firstcollector 20 formed over the first subcollector 18, the first base 22formed over the first collector 20, the first emitter 24 formed over thefirst base 22, and the first cap structure 26 formed over the firstemitter 24. The first cap structure 26 includes the first lower cap 34formed over the first emitter 24, the first middle cap 36 formed overthe first lower cap 34, the first ballast resistor layer 38 formed overthe first middle cap 36, and the first upper cap 40 formed over thefirst ballast resistor layer 38.

The second no-contact HBT 16NC has a similar configuration to the firstno-contact HBT 14NC. The second no-contact HBT 16NC includes the secondsubcollector 42, the second collector 44 formed over the secondsubcollector 42, the second base 46 formed over the second collector 44,the second emitter 48 formed over the second base 46, and the second capstructure 50L formed over the second emitter 48. The second capstructure 50L includes the second lower cap 58 formed over the secondemitter 48, the second middle cap 60 formed over the second lower cap58, the second ballast resistor layer 62 formed over the second middlecap 60, and the second upper cap 64 formed over the second ballastresistor layer 62. Herein, the second ballast resistor layer 62 may beformed from any lattice-match material to InGaAs with a wide bandgap.The second ballast resistor layer 62 may have a low doping concentrationbetween 1e17/cm³ and 1e19/cm³, and be formed of In_(0.5)Al_(0.5)As, InPor In_(y)Ga_((1-y))As. Herein, ‘y’ represents relative quantities ofIndium element. In this embodiment, ‘y’ having a value less than 0.5 mayprovide the second ballast resistor layer 62 a wider bandgap withadditional constraints of engineering the composition and thickness tomake the second ballast resistor layer 62 pseudomorphic.

Next, the first collector contacts 28 and the first base contacts 30 areprovided to the first no-contact HBT 14NC to form a firstno-emitter-contact HBT 14NEC, and the second collector contacts 52 andthe second base contacts 54 are provided to the second no-contact HBT16NC to form a second no-emitter-contact HBT 16NEC as depicted in FIG.6B. The first collector contacts 28 are formed over the firstsubcollector 18, and the first base contacts 30 are formed over thefirst base 22. The second collector contacts 52 are formed over thesecond subcollector 42, and the second base contacts 54 are formed overthe second base 46. The first emitter contact 32 is provided to thefirst no-emitter-contact HBT 14NEC to form the first HBT 14 as depictedin FIG. 6C. The emitter contact 32 is formed over the upper cap 40 ofthe first HBT 14.

Finally, the lengthened second emitter contact 56L is provided to thesecond no-emitter-contact HBT 16NEC to form the second HBT 16L asdepicted in FIG. 6D. The lengthened second emitter contact 56L extendsthrough the second upper cap 64 and the second ballast resistor layer 62of the second cap structure 50L, and extends into the second middle cap60. The lengthened second emitter contact 56L may be formed from alloyedmetal such that the second upper cap 64 and the second ballast resistorlayer 62 are shorted, and the resistances from the second upper cap 64and the second ballast resistor layer 62 are substantially eliminated.The lengthened second emitter contact 56L may be provided by an alloyingprocess. Alternatively, a trench may be etched by a wet or dry chemistryto reach into the second middle cap 60, and then the lengthened secondemitter contact 56L is placed into the trench.

FIGS. 7A-7C provide exemplary steps that illustrate a process tofabricate the exemplary semiconductor device 10NB shown in FIG. 4.Although the exemplary steps are illustrated in a series, the exemplarysteps are not necessarily order dependent. Some steps may be done in adifferent order than that presented. Further, processes within the scopeof this disclosure may include fewer or more steps than thoseillustrated in FIGS. 7A-7C.

Initially, a first no-contact HBT 14NB-NC and a second no-contact HBT16NB-NC formed over the substrate 12 are provided as depicted in FIG.7A. The first no-contact HBT 14NB-NC includes the first subcollector 18,the first collector 20 formed over the first subcollector 18, the firstbase 22 formed over the first collector 20, the first emitter 24 formedover the first base 22, and a first before-etching cap structure 26NB′formed over the first emitter 24. The first before-etching cap structure26NB′ includes the first lower cap 34 formed over the first emitter 24,the first middle cap 36 formed over the first lower cap 34, and thefirst upper cap 40 formed over the first middle cap 40. Notice thatthere is no ballast resistor layer inside the first before-etching capstructure 26NB′. The first middle cap 36 may be formed from GaAscompositionally graded into Indium-Gallium-Arsenide (InGaAs) from abottom to a top of the first middle cap 36 and may be used as an etchedstop layer in a later step.

The second no-contact HBT 16NB-NC has a similar configuration to thefirst no-contact HBT 14NB-NC. The second no-contact HBT 16NB-NC includesthe second subcollector 42, the second collector 44 formed over thesecond subcollector 42, the second base 46 formed over the secondcollector 44, the second emitter 48 formed over the second base 46, andthe second cap structure 50NB formed over the second emitter 48. Thesecond cap structure 50NB includes the second lower cap 58 formed overthe second emitter 48, the second middle cap 60 formed over the secondlower cap 58, and the second upper cap 64 formed over the second middlecap 60. There is no ballast resistor layer inside the second capstructure 50NB.

Next, the first upper cap 40 and the first middle cap 36 are removedfrom the first no-contact HBT 14NB-NC to form a first etched HBT 14NBEas depicted in FIG. 7B. The first lower cap 34 is included in the firstcap structure 26NB of the first etched HBT 14NBE. A wet/dry etchantchemistry, such as KOH, NaOH, and ACH, may be used to remove the firstupper cap 40 and the first middle cap 36. Because the first middle cap36 and the first upper cap 40 are highly doped with a dopingconcentration greater than 1 e19/cm³, the first lower cap 34 alone has ahigher contact resistance than the first before-etching cap structure26NB′, which combines the first lower cap 34, the first middle cap 36,and the first upper cap 40. Further, because the first lower cap 34 andthe second lower cap 58 may be formed from a common lower cap layer, thefirst middle cap 36 and the second middle cap 60 may be formed from acommon middle cap layer, and the first upper cap 40 and the second uppercap 64 may be formed from a common upper cap layer, the first lower cap34 alone may have a higher contact resistance than the second capstructure 50NB, which combines the second lower cap 58, the secondmiddle cap 60, and the second upper cap 64.

Finally, the first collector contacts 28, the first base contacts 30,and the first emitter contact 32 are provided to the first etched HBT14NBE to form the first HBT 14NB, and the second collector contacts 52,the second base contacts 54, and the second emitter contact 56 areprovided to the second no-contact HBT 16NB-NC to form the second HBT16NB as depicted in FIG. 7C. For the first HBT 14, the first collectorcontacts 28 are formed over the first subcollector 18, the first basecontacts 30 are formed over the first base 22, and the first emittercontact 32 is formed over the first lower cap 34 of the first capstructure 26NB. For the second HBT 16, the second collector contacts 52are formed over the second subcollector 42, the second base contacts 54are formed over the second base 46, and the second emitter contact 56 isformed over the second upper cap 64 of the second cap structure 50NB.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. An apparatus comprising: a substrate; a firstheterojunction bipolar transistor (HBT) formed over the substrate andcomprising: a first collector; a first base formed over the firstcollector; a first emitter formed over the first base; a first capstructure formed over the first emitter; and a first emitter contactconnected to the first cap structure; and a second HBT formed over thesubstrate and comprising: a second collector; a second base formed overthe second collector; a second emitter formed over the second base; asecond cap structure formed over the second emitter; and a secondemitter contact connected to the second cap structure, wherein the firstcap structure is different from the second cap structure, such that afirst emitter ballast resistance between the first emitter and the firstemitter contact is at least 1.5 times greater than a second emitterballast resistance between the second emitter and the second emittercontact.
 2. The apparatus of claim 1 wherein the first collector and thesecond collector are formed from a common collector layer, the firstbase and the second base are formed from a common base layer, and thefirst emitter and the second emitter are formed from a common emitterlayer.
 3. The apparatus of claim 1 wherein the first cap structurecomprises: a first lower cap formed over the first emitter; a firstmiddle cap formed over the first lower cap; a first ballast resistorlayer formed over the first middle cap; and a first upper cap formedover the first ballast resistor layer, wherein the first emitter contactis formed over the first upper cap.
 4. The apparatus of claim 3 whereinthe second cap structure comprises: a second lower cap formed over thesecond emitter; and a second middle cap formed over the second lowercap, wherein the second emitter contact is formed over the second middlecap.
 5. The apparatus of claim 4 wherein the first lower cap and thesecond lower cap are formed from a common lower cap layer, and the firstmiddle cap and the second middle cap are formed from a common middle caplayer.
 6. The apparatus of claim 4 wherein the first ballast resistorlayer has a doping concentration less than 1 e19/cm³ and comprises onefrom a group consisting of Indium-Aluminum-Arsenide (InAlAs),Indium-Phosphide (InP), and Indium-Gallium-Arsenide (InGaAs).
 7. Theapparatus of claim 6 wherein the first ballast resistor layer has athickness between 10 Å and 1500 Å.
 8. The apparatus of claim 6 wherein:the first lower cap and the second lower cap comprise Gallium Arsenidewith a doping concentration between 1 e17/cm³ and 1 e19/cm³; the firstmiddle cap and the second middle cap comprise Gallium-Arsenidecompositionally graded into Indium-Gallium-Arsenide with a dopingconcentration greater than 1 e19/cm³; and the first upper cap comprisesIndium-Gallium-Arsenide with a doping concentration greater than 1e19/cm³.
 9. The apparatus of claim 3 wherein the second cap structurecomprises: a second lower cap formed over the second emitter; a secondmiddle cap formed over the second lower cap; a second ballast resistorlayer formed over the second middle cap; and a second upper cap formedover the second ballast resistor layer, wherein the second emittercontact extends through the second upper cap and the second ballastresistor layer, and extends into the second middle cap.
 10. Theapparatus of claim 9 wherein the first lower cap and the second lowercap are formed from a common lower cap layer, the first middle cap andthe second middle cap are formed from a common middle cap layer, and thefirst upper cap and the second upper cap are formed from a common uppercap layer.
 11. The apparatus of claim 9 wherein: the first ballastresistor layer has a doping concentration less than 1e19/cm³ andcomprises one from a group consisting of InAlAs, InP, and InGaAs; andthe second ballast resistor layer has a doping concentration less than 1e19/cm³ and comprises one from a group consisting of InAlAs, InP, andInGaAs.
 12. The apparatus of claim 11 wherein the first ballast resistorlayer has a thickness between 10 Å and 1500 Å, and the second ballastresistor layer has a thickness between 10 Å and 1500 Å.
 13. Theapparatus of claim 11 wherein: the first lower cap and the second lowercap comprise Gallium Arsenide with a doping concentration between 1e17/cm³ and 1 e19/cm³; the first middle cap and the second middle capcomprise Gallium-Arsenide compositionally graded intoIndium-Gallium-Arsenide with a doping concentration greater than 1e19/cm³; and the first upper cap and the second upper cap compriseIndium-Gallium-Arsenide with a doping concentration greater than 1e19/cm³.
 14. The apparatus of claim 1 wherein the first cap structurecomprises a first lower cap formed over the first emitter, and the firstemitter contact is formed over the first lower cap.
 15. The apparatus ofclaim 14 wherein the second cap structure comprises: a second lower capformed over the second emitter; a second middle cap formed over thesecond lower cap; and a second upper cap formed over the second middlecap, wherein the second emitter contact is formed over the second uppercap.
 16. The apparatus of claim 15 wherein the first lower cap and thesecond lower cap are formed from a common lower cap layer.
 17. Theapparatus of claim 15 wherein: the first lower cap and the second lowercap comprise Gallium Arsenide with a doping concentration betweenbetween 1e17/cm³ and 1 e19/cm³; the second middle cap comprisesGallium-Arsenide compositionally graded into Indium-Gallium-Arsenidewith a doping concentration greater than 1e19/cm³; and the second uppercap comprises Indium-Gallium-Arsenide with a doping concentrationgreater than 1 e19/cm³.
 18. The apparatus of claim 15 wherein the secondmiddle cap and second upper cap 64 are comprised of a superlatticestructure.
 19. The apparatus of claim 1 wherein the first HBT furthercomprises a first subcollector between the substrate and the firstcollector and the second HBT further comprises a second subcollectorbetween the substrate and the second collector.
 20. The apparatus ofclaim 19 wherein the first subcollector and the second subcollector areformed from a common subcollector layer.
 21. The apparatus of claim 19wherein the first subcollector and the second subcollector are formedfrom a contiguous subcollector layer.
 22. The apparatus of claim 1wherein either the first HBT and the second HBT are NPN-type bipolartransistors or the first HBT and the second HBT are PNP-type bipolartransistors.
 23. A method comprising: providing a first no-contact HBTand a second no-contact HBT formed over a substrate, wherein the firstno-contact HBT comprises a first lower cap over a first emitter, a firstmiddle cap over the first lower cap, a first ballast resistor layer overthe first middle cap, and a first upper cap over the first ballastresistor layer at a top portion of the first no-contact HBT, and thesecond no-contact HBT comprises a second lower cap over a secondemitter, a second middle cap over the second lower cap, a second ballastresistor layer over the second middle cap, and a second upper cap overthe second ballast resistor layer at a top portion of the secondno-contact HBT; removing the second upper cap and the second ballastresistor layer from the second no-contact HBT to form a second etchedHBT; and providing a first emitter contact to the first no-contact HBTto form a first HBT, wherein the first emitter contact is formed overthe first upper cap, and providing a second emitter contact to thesecond etched HBT to form a second HBT, wherein the second emittercontact is formed over the second middle cap.
 24. A method comprising:providing a first no-contact HBT and a second no-contact HBT formed overa substrate, wherein the first no-contact HBT comprises a first lowercap over a first emitter, a first middle cap over the first lower cap, afirst ballast resistor layer over the first middle cap, and a firstupper cap over the first ballast resistor layer at a top portion of thefirst no-contact HBT, and the second no-contact HBT comprises a secondlower cap over a second emitter, a second middle cap over the secondlower cap, a second ballast resistor layer over the second middle cap,and a second upper cap over the second ballast resistor layer at a topportion of the second no-contact HBT; providing a first emitter contactto the first no-contact HBT to form a first HBT, wherein the firstemitter contact is formed over the first upper cap; and providing asecond emitter contact to the second no-contact HBT to form a secondHBT, wherein the second emitter contact extends through the second uppercap and the second ballast resistor layer, and extends into the secondmiddle cap.
 25. A method comprising: providing a first no-contact HBTand a second no-contact HBT formed over a substrate, wherein the firstno-contact HBT comprises a first lower cap over a first emitter, a firstmiddle cap over the first lower cap, and a first upper cap over thefirst middle cap at a top portion of the first no-contact HBT, and thesecond no-contact HBT comprises a second lower cap over a secondemitter, a second middle cap over the second lower cap, and a secondupper cap over the second middle cap at a top portion of the secondno-contact HBT; removing the first upper cap and the first middle capfrom the first no-contact HBT to form a first etched HBT; and providinga first emitter contact to the first etched HBT to form a first HBT,wherein the first emitter contact is formed over the first lower cap,and providing a second emitter contact to the second no-contact HBT toform a second HBT, wherein the second emitter contact is formed over thesecond upper cap.